A suspected break in the core of a nuclear reactor could have been responsible for a leak of large amounts of radioactive contamination at the Fukushima Daiichi power plant, Japanese nuclear safety officials said on Friday, in another setback to efforts to avert disaster at the stricken facility.

In the latest developments, officials have said seawater outside one of the units has registered 1,250 the normal level of radiation, while efforts are under way to pump radioactive water that has pooled around the reactor turbines into safe storage. The BBC has reported that short-term radioactive iodine has been detected at very high levels in the Pacific Ocean near the plant.

US naval barges have started rushing in supplies of fresh water amid concerns the seawater being used to cool down the reactors might be causing corrosion.

The Japanese prime minister, Naoto Kan, described the situation at the plant as "very serious", while media reports said that two men who were injured while working on the plant's No.3 reactor on Thursday may have suffered internal exposure to radiation.

"We must remain vigilant," Kan told a televised news conference. "We are doing our best to prevent a deterioration in the situation, but we are not yet in a position that allows us to be optimistic. We must treat every development with the utmost care."

He praised the hundreds of Tokyo Electric Power employees, troops, firefighters and police officers who are struggling to avert disaster at the plant, and insisted any new information would be released quickly to the Japanese people and the international community.

"We have been transparent right from the start and will continue to give accurate information about radiation levels and their possible impact on health," he said.

Growing uncertainty over radiation contamination led to an increased exodus from the area surrounding Japan's stricken nuclear plant.

Officials in towns across Fukushima prefecture said they had been instructed to prepare more shelters in anticipation of a new influx of evacuees.

The Japanese government said it was considering extending its evacuation order to a 30km (19 mile) radius of the damaged Fukushima reactors. In Nihonmatsu, which is already housing 2,900 people who fled homes close to the reactor, officials said they were advised to prepare 10 new shelters for the next wave of nuclear evacuees.

"We are preparing for more evacuations," said Hatori Norio, who serves on the town's disaster committee.

The town is also preparing for the possibility that the evacuees will stay for the duration. He said the town had begun putting evacuees on the waiting list for public housing, recognizing that it may be years before they return to their homes.

The government's chief spokesman, Yukio Edano, said 130,000 residents in the area had been encouraged to leave to improve their quality of life, not because their health was at risk. The nuclear emergency, 150 miles north of Tokyo, has caused severe disruption to business, supply routes and other services in the area.

On Thursday three workers were exposed to unusually high levels of radiation after stepping in contaminated water in the turbine building of the crippled No. 3 reactor, which they were trying to cool.

Two received possible beta ray burns to their legs. All three have been transferred to a special radiation treatment facility. Kyodo news reported that the two more seriously injured workers could have suffered internal radiation exposure.

"The contaminated water had 10,000 times the amount of radiation as would be found in water circulating from a normally operating reactor," said Hidehiko Nishiyama, a spokesman for Japan's nuclear safety agency.

Nishiyama said it was unlikely that the reactor had cracked, but conceded that the unusually high levels of radiation appeared to have originated from its core. "It is possible there may be damage somewhere in the reactor," he said, adding that a leak in the plumbing or the vents could also be to blame.

Hideo Morimoto, director of Japan's agency for natural resources and energy, said the water leak did not mean the battle to prevent meltdown at the reactor was being lost. "I feel that if the pressure vessel has been seriously damaged, then far more radiation would have leaked," he said.

The temperature and pressure inside the core, which holds the fuel rods, remained stable and was far too low to be capable of further melting the core, officials said.

Dr Ian Haslam, head of radiation protection at Leeds University, said there was a "real possibility" a hydrogen explosion at the reactor on 14 March had damaged the containment vessel and the fuel rods.

But he added: "If there was significant damage to the rods and significant holes, the contamination would be a lot higher; we would be seeing a lot more steam coming out."

Edano said the source of the leak remained unknown. "We are exploring every possibility, but we don't think this is a new situation, rather that a certain amount of radiation may have leaked from the reactor. This is a possibility that we have been mentioning for some time.

"But at this point we don't know if the radiation is coming from the reactor itself or from another source."

Nuclear officials say the leak may have come from pipes or the reactor's pool for storing spent fuel rods, which workers have been struggling to cool since the plant was badly damaged in the 11 March earthquake and tsunami.

The reactor contains 170 tons of radioactive fuel in its core, and is the only one of the facility's six reactors that contains the potentially more dangerous plutonium-uranium mixed oxide fuel.

Thursday's accident forced a temporary halt to work on two reactors while technicians check radiation levels.

"We should try to avoid delays if at all possible, but we also need to ensure that the people working there are safe," Nishiyama said.

The official death toll from the earthquake and tsunami has risen above 10,000, according to the national police agency. But with more than 17,400 listed as missing, the final number of dead is expected to rise significantly.